1. Field of the Invention
The present invention relates to a method of manufacturing a clad bar comprising an inner layer of one metal and an outer layer formed of another metal.
2. Description of the Prior Art
A clad bar comprising a core member and an outer layer member coated on an outside of said core member to utilize mechanical properties of the core member and a corrosion-resistance, abrasion-resistance and beautiful external appearance of the outer layer member has been known. The following methods of manufacturing a clad bar have been known.
&lt;1&gt; Japanese Patent Laid-Open No. 141313/1980
This relates to a method in which a core member is fitted in a cylindrical outer layer member, the resulting assembly being subjected to a cold drawing to closely contact the outer layer member to the core member, and then the cold drawn assembly being heated followed by rolling by grooved rolls. With this method, a brittle layer of intermetallic compounds is formed at the bonding interface between the core member and the outer layer member, whereby the sufficient bond strength cannot be attained.
&lt;2&gt; Japanese Patent Laid-Open No. 160551/1979
This relates to a method in which a core member is fitted in a cylindrical outer layer member, the resulting assembly being subjected to a cold drawing, and then annealed to bring about the diffusion through the boundary surface, whereby carrying out the bond. With this method, since intermetallic compounds formed by the diffusion are brittle and weak, the bond strength is reduced.
&lt;3&gt; Japanese Patent Laid-Open No. 110486/1984
This relates to a method in which a core member is fitted in a cylindrical outer member, the resulting assembly being subjected to a cold reduction, a disk formed of the same material as the outer layer member being welded to both end faces of the reduced assembly by the friction welding to seal up a gap between the core member and the outer layer member, and then the assembly being heated followed by being subjected to a hot rolling by grooved rolls or hot extrusion.
With this method, the rolling is alternately carried out in a direction different 90.degree. to each other in the hot rolling by the grooved rolls, so that a portion subjected to the compression in one rolling receives a tensile force in a radial direction in the subsequent rolling, whereby bringing out the separation of the outer layer member from the core member at the bonding interface therebetween. In addition, the hot extrusion does not lead to the attainment of the sufficient bond strength.
&lt;4&gt; Japanese Patent Laid-Open No. 103928/1983
This relates to a method in which a core member is fitted in a cylindrical outer layer member, and then merely the outer layer member is reduced by means of a die so that the core member may not be deformed. With this method, since a heating is not applied, a diffusion layer is not formed in the bonding interface between the core member and the outer layer member, that is, the core member and the outer layer member are not integrated with each other. As a result, the bond strength is reduced.
&lt;5&gt; Japanese Patent Publication No. 8188/1979
This relates to a method in which a core member is fitted in an outer layer member, and then both members are simultaneously elongated by the hydrostatic extrusion method to carry out the bond. With this method, not only the bond strength is not sufficient, but also a length of a product capable of manufacturing has an upper limit since it is necessary to increase an elongation rate in the event that a long product is manufactured. In addition, this method is complicated in comparison with the methods &lt;1&gt; to &lt;4&gt;.
Besides, in a rolling method using a grooved roll as in the methods &lt;1&gt; and &lt;3&gt;, a sectional shape of the core member after rolling becomes quite different from a circular shape, so that a thickness of the outer layer member becomes uneven. Accordingly, disadvantages occur in the exposure of the core member in the subsequent turning process and the like.
As described above, with the conventional methods, no sufficient bond strength has been attained. Accordingly, the development of a method of manufacturing a clad bar, to which a superior bond strength is required, has been expected.